Experimental Studies of the difference between electron and ion densities measured by Langmuir probes in the presheath

It has recently been shown[1] that Langmuir Probes (LPs) measure an unphysically positive plasma potential in the presheath of low temperature plasma, near conducting boundaries at which ion rich sheaths form, compared with the plasma potential measured by emissive probes in the region. We present[2] experimental evidence that concomittant with this, a similar and related difference occurs between electron and ion densities Langmuir probes in the presheath. A negatively biased plate (-100V) is immersed in a weakly collisional (λ_mpf «λ_D), low pressure (P_n ≤1 mTorr), low temperature (kT_e ~ 1eV), single ion species plasma formed in a hot-filament DC discharge, where the feedstock gas is argon, helium, or krypton. Here we present details of the electron and ion density profile measurements, which differed in the presheath, that were designed to test whether the ion flow in the presheath, quantified by the calculate Bohm speed, V_B = √(kT_e/m_i ), affects the difference between electron and ion densities.

[1] P. Li et al. PSST {f 29} (2020) 025015}